1. Field of the Invention
This invention relates to an undercoat composition with excellent adhesiveness to both a resin coating layer and a metal layer. It also relates to a metal substrate coated with a resin composition and more particularly to a resin-coated metal substrate with excellent hot-water resistance and chemical resistance.
2. Description of the Prior Art
Various metal substrates are coated with resin compositions to prevent rusting and to improve heat resistance and chemical resistance.
A polyaryletherketone resin and a polyphenylenesulfide resin are crystalline resins with excellent mechanical properties and chemical properties. Also, they have excellent heat resistance, water resistance, and corrosion resistance. Therefore, these resins are used as insulating materials for electric wires and cables, injection-molded articles, composites made of films or monofilaments, and the like. For example, Japanese Laid-open Patent Publication No. 56-92952 discloses a composition comprising a polyaryletherketone resin, glass, and/or a carbon reinforcing material. However, when these resins are used as a coating material for a metal substrate to obtain a non-crystalline resin coating layer formed on the metal substrate, water and chemical solutions are much diffused in the resin coating layer. Thus, when the resultant coated metal substrate comes into contact with water and chemical solutions, the resin coating layer is quite susceptible to the generation of blisters, and the coated metal substrate does not have satisfactory heat resistance or chemical resistance. Therefore, attempts have been made to cause these resins to be crystallized after their application to the metal substrate. In the resin coating layer containing the crystallized resins, water and chemical solutions are not much diffused. Therefore, the heat resistance and chemical resistance of the resultant coated metal substrate are improved. However, the resin coating layer is quite susceptible to cracking because of shrinkage thereof caused by the crystallization of the resin, resulting in poor impact resistance.
On the other hand, a polyarylethersulfone resin is excellent in heat resistance, chemical resistance, and workability, as well as in mechanical properties such as wear resistance and impact resistance. Also, the polyarylethersulfone resin is chemically stable in the range of from ordinary temperatures to the glass transition point. Moreover, because the polyarylethersulfone resin is a non-crystalline resin, when the polyarylethersulfone resin is used as a coating material for metal substrates, residual stress caused by the crystallization of the resin in the resultant coating layer does not occur.
Japanese Laid-Open Patent Publication No. 54-34335 discloses a powder coating method using a polyarylethersulfone resin. In this method, a resin coating layer formed on a metal substrate is treated at a given temperature and then baked, so that the adhesiveness of the resin coating layer to the metal substrate is improved. Therefore, the resin coating layer is relatively less susceptible to the generation of blisters and peeling in ordinary applications. However, because the polyarylethersulfone resin has relatively high water absorbing capacity, when the resultant coated metal substrate comes into contact with steam at a high temperature, the resin coating layer is quite susceptible to the generation of blisters and peeling.
Also, when resin compositions containing the above-mentioned resins are used for coating a metal substrate, the metal substrate is first treated by, for example, alkaline treatment to improve the adhesiveness of the resin coating layer to the metal substrate.
For the alkaline treatment, zinc phosphate and the like are used. However, the layer formed by the alkaline treatment does not have satisfactory heat resistance. Therefore, when the resin composition is used for coating the metal substrate that has been treated by alkaline treatment, and then the resin coating layer is baked, the layer formed by the alkaline treatment is degraded because of the application of heat. As a result, the adhesiveness of the resin coating layer to the metal substrate becomes poor.
To solve these problems, there has been proposed a method of coating a metal substrate with an undercoat composition. Japanese Patent Publication No. 59-44912 discloses various organic primers such as polyamide-imide resins and polyimide resins as undercoat compositions that are useful in coating a metal substrate with a fluororesin. However, these organic primers hardened in the step of baking do not have satisfactory adhesion to a resin coating layer formed thereon. Therefore, when the coated metal substrate obtained by undercoating a metal substrate with these organic primers comes into contact with steam at a high temperature, the adhesiveness of the resin coating layer to the metal substrate becomes poor.
As the undercoat composition, there are well-known mixtures of fine-powdered zinc and heat-stable binders such as polyalkylsilicates (disclosed in Japanese Laid-Open Patent Publication No. 49-128836) and compositions comprising powdered aluminium in granular form, powdered zinc in granular form, and the condensate of alkylsilicates (disclosed in Japanese Laid-Open Patent Publication No. 54-29340). An undercoat composition comprising polyalkylsilicates and powdered metal such as powdered aluminium provides relatively good adhesiveness to both a metal substrate and a resin coating layer made of a fluororesin or a polyphenylenesulfide resin. However, an undercoat layer containing an inorganic binder such as polyalkylsilicate does not have satisfactory adhesion to a resin coating layer. Therefore, when the resultant coated metal substrates obtained by the use of these undercoat compositions come into contact with steam at a high temperature, the adhesiveness of the resin coating layer to the metal substrate becomes poor.